hello ,
i too use Webpagemaker 2.5 for my webdesigning.
this is simple.
open WPM v2 -> click on 'insert' tab above and then click 'HTML CODES AND SCRIPTCODE' button.
you can insert your (php script i.e the login script). be sure to save the file as file.php . the extension .php is important whjile publishing the pages.
to make the script easy, you can do login system which doesnot use mysql database. i have many scripts & i can send you if u wish to ask me